"CW" refers to the new vacuum-tube Continuous Wave transmitters, which were rapidly supplanting spark transmitters, due to their better efficiency and quieter operation.
 
QST, August, 1921, page 32:
The  Transition
IN every mail we get news of another good spark station gone wrong--the apparatus junked or offered for sale. "I'm putting in a good C.W. set", the writers say, "and I'll be all ready for business by fall."
    That simple little story is being repeated in every town every day. Men, we have reached the great transition--the dawn of the change we have been expecting. The spark is not "doomed"; not a bit of it; we will have sparks with us for years to come. But co-operation, if nothing else, will demand that they be improved in their interfering proclivities and only the really good ones will survive. And the mediocre spark station will surely pass, for the simple reason that it can be easily surpassed by a small C.W. set. Thus by fall we expect that the situation will be pretty much this: the average DX station will be C.W., and the DX sparks will be those few really good ones whose owners are satisfied with their range and hesitate to venture into a new field.
    Meanwhile traffic suffers. To be blunt, traffic work is shot all to the devil, what with rotten weather and experimenting with C.W. This period has been foreseen, tho, and we are fortunate that it has come upon us in the summer when traffic is naturally light, and doubly fortunate in that we will be all set for traffic by cool weather. A.R.R.L. members will not forget that our main work is the DX handling of citizen messages, and we want to work hard, all of us, to get the C.W. sets whipped into shape so that our traffic can rise to its normal proportions with the coming of fall.
    It certainly is a comfortable feeling to have a good C.W. transmitter. It will work at wave lengths so short that a spark set wouldn't have any output worth mentioning if it tried to get down as low. It goes jumping thru the static when the sparks are helpless. It's compact and quiet, and Oh Brother what distances per watt output! The experiences of those actively on the air with C.W. sets has amply proved that our faith in the quiet efficiency of tube transmission was well founded, and the earnest consideration of these facts is asked of the dwindling minority of "spark hounds".
    So here's the word, fellows: traffic is suffering, as suffer it must while the change is on, but let's make it short and snappy and bend every effort to getting the C.W. sets (and that very positively includes receiving sets) lined up for DX work by the time September rolls around.
    And next winter we'll have an All-CW Transcon Route! Will you be on it?
Page 34:
The  Operating  Department

F.  H.  Schnell,  Traffic Manager

One thing that stands out more prominently than any other is the fact that spark stations are being dismantled and sold while C.W. sets are taking their places very conspicuously. The entire amateur world seems to be undergoing a change. CW, CW, CW is all one hears these days and the fall and winter will not find all of those famous long distance spark stations but in their stead will be the little squeal of the CW reaching out over greater distances than those old familiar sparks that one could recognize before the call was signed. Those familiar sparks did handle messages by the hundreds, but CW will be the means of better and more reliable communication. We can be sure of less QRM from sparks as, at the present rate of dismantling, there will be but few left.